This invention relates to a method for coating micro-electromechanical systems (MEMS) devices.
Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through the utilization of microfabrication technology. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible xe2x80x9cmicromachiningxe2x80x9d processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices on size scale of from 1 to 1000 microns. Examples of MEMS devices include airbag accelerometers (F. Goodenough, Electronic Design 8, 45 (1991)), gyros (R. T. Howe, B. E. Boser, and A. P. Pisano, Sens. Acturators A 56, 167 (1996)), optical switches, movable diffraction gratings (U.S. Pat. No. 6,233,087), and inkjet printheads (U.S. Pat. No. 6,079,821).
Due to the physical nature of the miniature structural members, such as the large surface-to-volume ratio and small offset from adjacent surfaces, MEMS devices are especially vulnerable to adhesion upon contact. Therefore, the xe2x80x9cstictionxe2x80x9d problem remains a major limitation in bringing new MEMS devices to the broader market (M. P. de Boer, T. A. Michalske, J. Applied Physics, 86, 817 (1999)). It is found that coatings reducing the surface energy of the miniature structural members, particularly those based silane coupling agents, have shown some promise for reducing stiction and improving MEMS performance (R. Maboudian, R. T. Howe, J. Vac. Sci. Tech. B, 15, 1 (1997)). However, the processes of applying silane coupling coating agents are generally involving with some liquid solvents (E. P. Pleuddemann, Silane Coupling Agents, Plenum Press, New York, 1982). Those xe2x80x9cwetxe2x80x9d processes are generally not desirable for MEMS fabrication as they cause problems in structure releasing. On the other hand, due to the high volatility, low flashpoint, and high instability (causing self-polymerization) of most silane coupling agents, a well controlled direct chemical vapor deposition process which results in repeatable uniform coatings is very difficult to achieve.
Another important class of MEMS are microfluidic devices, such an inkjet drop ejector. In the many of the microfluidic MEMS applications, coatings that control the surface wetting properties are especially important. U.S. Pat. Nos. 4,343,013; 4,555,062; and 4,623,906 and European Patents 117,316; 92,229; and 121,623 disclose the applications of anti-wetting coatings on the nozzle plates of inkjet printheads.
A MEMS inkjet drop ejector or printhead is formed out of silicon wafers using orientation-dependent etching techniques to create an array of nozzles on the orifice plate. With modern MEMS technology, various thermal and logic transducer components, known as heaters and logic control drivers, can be integrated onto the nozzle plates with designed microstructures located around nozzle orifices. The top surface of such fabricated printheads is composed of silicon oxide which may additionally be coated with a passivation layer of a metallic oxide such as tantalum oxide or a metallic nitride such as silicon nitride.
U.S. Pat. No. 5,598,193 discloses a treatment of the outer surface of a gold-plated nozzle plate with thiols, disulfides, or sulfinates to form a monolayer coating chemically bonded to the gold surface. U.S. Pat. Nos. 4,643,948 and 5,010,356 disclose coating methods of coating partly fluorinated alkyl silanes on the surfaces of inkjet printheads. It is disclosed by U.S. Pat. No. 6,345,881 to Yang et al. that a stamp or pad may be used to apply the coating to the nozzle plate. However, there is a problem with these methods in that they employ solvents that cause a capillary force induced stiction, or autoadhesion. In addition, those silanes often self-polymerize in solvents resulting in non-uniform coatings with large aggregates and particles that adversely affect the performances for the printheads.
An object of this invention is to provide an improved method for coating the surfaces of a MEMS device with a silane coupling agent in order to reduce autoadhesion. This and other objects are achieved in accordance with the present invention that comprises a method for coating a micro-electromechanical systems device with a silane coupling agent by
a) mixing the silane coupling agent with a low volatile matrix material in a coating source material container;
b) placing the micro-electromechanical systems device in a vacuum deposition chamber that in connection with the coating source material container;
c) pumping the vacuum deposition chamber to a predetermined pressure; and maintaining the pressure of the vacuum deposition chamber for a period of time in order to chemically vapor deposit the silane coupling agent on the surface of the micro-electromechanical systems device
By use of the invention, coatings of MEMS devices are achieved by a well controlled process, and the coatings applied by the method of the invention are very uniform, free of defects, aggregates, and particle contamination. In addition, anti-stiction coatings are effectively achieved on the actuating MEMS devices, and the anti-wetting coating is also effectively applied to the surface of a MEMS inkjet drop ejector. Further, by use of the invention the coating agent mixed in the matrix materials evaporates much slower under vacuum, thus resulting in a more uniform self-assembled monolayer (SAM) coating.